The present invention relates to an ignition device for internal combustion engines, comprising a coil which includes a primary winding and a secondary winding and which is provided with an armature in which the passage of a magnet, driven by the rotation of the engine, produces a variable magnetic flux which induces in the primary winding a current pulse whose interruption causes a high-voltage pulse for a spark plug in the secondary winding, a primary current circuit in which said current is restored and which comprises an impedance in series with an interrupt transistor, and a circuit for controlling the turning on and off of said interrupt transistor, causing the interruption of the current by the turning off of said interrupt transistor when the value of a control signal applied to an input terminal of this circuit exceeds a given threshold value.
A device of this kind is used in small internal combustion engines, for example for lawn mowers, chain saws, hedge trimmers, etc.
A device of this kind is known from German Patent DE 23 14559. The circuit described in the cited document comprises a Darlington-type NPN interrupt transistor with an impedance which is connected in series in its emitter connection and which is in this case a current measuring resistance. The voltage generated by the current in said resistance is applied to the base-emitter region of an NPN transistor which is thus turned on when the current exceeds a given value. This transistor is connected so that its turned-on state turns off the interrupt transistor.
Such a device functions, but is not protected against operation of the engine in the reverse direction.